Optical amplifiers, and more specifically optical fibre amplifiers, are used in many applications. Such amplifiers use a gain medium into which a pump light is guided to amplify a signal guided within the gain medium. However, these amplifiers typically have numerous drawbacks. For example, it is relatively difficult to achieve large gains in such amplifiers as there are typically problems caused by the amplification of spontaneous light emission and saturation of the amplifying medium at large gains.
These problems are usually circumvented in prior art devices by concatenating many optical amplifiers having a relatively small amplification gain. However, there is typically a need to separate the various amplifier stages by many optical components such as optical isolators and optical filters to prevent the build-up of amplified scattering noise in the amplifier and back propagation of potentially damaging light. In turn, this causes these conventional optical fibre amplifiers to be relatively complex and relatively costly. In addition, the use of many components increases losses in signal power as even good quality components typically all have non-negligible power losses. Furthermore, there is typically a need to have relatively long optical fibres in each amplification state so that the light pumped in each stage is mostly absorbed before reaching the following further stage.
Many inventors have tried to improve on the basic design described hereinabove, but all these improved designs still have drawbacks. For example, PCT Application 2005/112206 published Nov. 24, 2005 and applied for by Sintov describes an optical amplifier made out of a doped optical fibre in which pump light is injected at many longitudinally spaced apart locations. Sintov explicitly mentions that a drawback or, at least, a characteristic of his amplifier is that there is a need for near complete absorption of light between injection points. In addition, in this amplifier, power is injected at similar levels at all locations. Therefore, achieving a large gain in this amplifier still requires that the amplifier be relatively large and, therefore, relatively costly to build.
In another example, U.S. Pat. No. 5,933,271 published Aug. 3, 1999 and issued to Waarts et al. describes an optical fibre in which pump light is injected in counter-propagating direction from both ends of the optical fibre. Once again, in this patent, the amplifier requires that the optical fibre be long enough so that most of the pump light injected at one end is absorbed before it reaches the other end. More specifically, Waarts even suggests that there is a need for most of the pump light to be absorbed before half of the fibre length. Also, while this patent shows how to increase gain in an optical amplifier by injecting light at two ends thereof, there is no manner in which pump power could be further increased, as optical fibres are essentially one-dimensional objects for this purpose as they only have two ends.
Against this background, there exists a need in the industry to provide an improved optical amplifier. An object of the present invention is therefore to provide such an optical amplifier and methods of assembling same.